Circular settling tanks are commonly used for treatment of water and wastewater. Water containing particles is introduced into the settling tank and the particles will settle to the bottom of the settling tank where they will form a sludge layer. The influent to the settling tank is divided into an overflow and an underflow. Settling tanks are expected to produce a reasonably clean overflow and a reasonably thick underflow. If the emphasis is on the clean overflow they are often named clarifiers and if the emphasis is on a thick underflow they are often named thickeners. The underflow normally leaves the settling tank through a central outlet in the bottom of the tank. The sludge that has collected on the tank bottom is transported to the underflow outlet by means of at least one sludge scraper. A sludge scraper normally consists of a scraper arm with one or more scraper blades attached. The scraper arm may be fixed to a central vertical shaft. The shaft is provided with a drive motor which rotates the shaft and scraper arm(s). Since the scraper blades are set at an angle relative to the scraper arm a transport of the sludge towards the center will result.
As an alternative to the center drive motor, the scraper arm can be driven from the periphery. In that case it is common to have a drive motor running on a driving wheel along the top of the tank wall. The peripheral motor pulls the scraper arm(s) and gives it a circular motion. The driving force is normally transferred to the scraper arm by a truss.
The peripheral drive can pull scrapers working with a heavy load of sludge, but it has certain disadvantages. For instance, in winter snow and ice will interfere with the operation, the peripheral drive makes it difficult to provide a roof over the settling tank and the truss makes it impossible to install inclined settler plate packs in the settling tank in order to increase settling capacity.
In a prior art system, a drive motor is provided at the periphery of the tank. Three vertical shafts extend vertically downwards to close to the bottom of the settling tank, and at least one shaft is connected to the motor. At the lower end of each shaft a cog wheel is provided. The cog wheels connect with a metal ring that is provided along the periphery of the settling tank, just above the bottom of the tank. The scraper arms are connected to the ring. The ring consists of two ring formed sections arranged one above the other. These sections are connected by a great number of metal pins each enclosed by a piece of plastic tube which can rotate freely around the pins and thereby functions as a bearing enabling the ring to be rotated by use of the cog wheels. Since the pins need to be placed close to each other to engage with the cogs of the cog wheels a great number of pins enclosed with plastic tubes will have to be provided, for a large settling tank up to 1000 pieces. In addition to being connected to the ring the scraper arms are also connected to a central rotating means.
A similar scraper drive exists wherein the metal ring has been exchanged for a ring built up by a number of plastic or rubber elements.